Exclude posts from child categories on archive pages

functions.php

<?php

/**
 * Filter the archive posts to only display posts of the current Term not child Terms.
 *
 * @param WP_Query $query The query instance.
 * @return void
 */
function filter_archive_posts( $query ) {

	if ( is_admin() || ! $query->is_main_query() || ! is_category() ) {
		return;
	}

	if ( isset( $query->tax_query ) ) {

		// Loop through current queries and exclude children.
		$tax_query = array_map( function( $q ) {
			$q['include_children'] = false;
			return $q;
		}, $query->tax_query->queries );

		// Set query and relation.
		$query->set( 'tax_query', [
			'relation' => $query->tax_query->relation,
			$tax_query,
		] );

	}
}
add_action( 'pre_get_posts', 'filter_archive_posts', 10 ,1 );